Technical Field
The present invention relates to a system and an apparatus for providing beamforming vectors for very high density cellular networks, and a corresponding method, a corresponding program and a corresponding recording medium. More specifically, a system, an apparatus, a method, a program and a recording media for providing beamforming vectors for wireless network systems that reduces complexity of the system by a one-shot, non-iterative, decision of beamforming vectors according to a predetermined decision metric called global selfishness is provided.
Background Art
Recently, due to unlimited data plans of telecommunication companies, traffic of mobile communication data has increased by an unprecedented rate. To meet this rapidly increasing traffic of mobile communication data, high-density cellular networks that have more and smaller cells are proposed, such as Femtocells. However, a gain of spatial reuse by using smaller cells fatally requires very high maintenance costs. As the network infrastructure is increasingly crowded with the addition of small cells, inter-cell interference (ICI) is getting worse, and the amount of information that requires computation is increasing in order to manage ICI is rapidly increasing. Therefore, a method for managing the ICI effectively while lowering the complexity is required.
More specifically, cooperative beamforming (BF) schemes using a multi-antenna system are used for a multiple access system in order to improve the performance of the system and increase a capacity. Typically, cooperative beamforming schemes refer to arranging a plurality of antennas at regular intervals, and transferring the signal to each antenna by applying weighting vector.
In cellular networks, ICI is one of the most dominating factors that determine the performance of cellular systems. The sum rate performance is significantly degraded by ICI, especially when a small number of frequency reuse factors are adopted in the network. Thus, there have been seamless efforts to efficiently reduce the ICI by introducing cooperative beamforming among the cells. A well-known coordinated multi-point (CoMP) technique is one example of this effort, which has been rigorously developed for commercial 3rd generation partnership project long term evolution (3GPP-LTE) systems. The main hurdle for the network-wide sum rate maximization is its mathematical intractability. It is very difficult to determine the cooperative beamforming vectors that maximize the desired signal power of one cell while minimizing the generated interference to other cells, since they are all coupled in terms of the sum rate. Unfortunately, no closed-form solution is known for the problem. Instead, some alternative approaches have been proposed in the literature (non-patent documents 3˜9) as described below.
Referring to the non-patent documents 3 and 4, a new matrix is defined as the ratio of the desired signal power and generated interference to neighboring cells and noise power (SGINR), and the cooperative beamforming vectors in each cell are individually determined to maximize it. The SGINR based cooperative beamforming considers both the maximization of the desired signal power and the minimization of the generated interference to neighbors, thus increasing the network-wide sum rate. For a two-cell case, it has been proved that the SGINR-based cooperative beamforming is equivalent to the optimal beamforming that maximizes the network-wide sum rate.
Zero forcing (ZF) beamforming can be used to obtain an optimal weight vector for each antenna. Zero forcing beamforming removes the interference signal by multiplying the inverse matrix of the channel in advance when transmitting the signals, so that no interference occurs to the other receivers that are not the target of the transmitted signals. Thus, zero forcing beamforming is regarded as altruistic beamforming. Alternately, maximal ratio transmission (MRT) is beamforming that focuses its power on the target receiver without considering interference to the other receivers. Therefore, maximal ratio transmission beamforming is regarded as egoistic beamforming.
Therefore, high transmission efficiency can be expected when using the egoistic beamforming, such as the maximum ratio transmission beamforming in the communication environment at the favorable channel condition, and when using the altruistic beamforming, such as zero-forcing beamforming in the communication environment in an inferior channel condition. However, the channel condition changes over time, so it is required to combine egoistic beamforming and altruistic beamforming properly in order to obtain the optimal transmission efficiency. Non-patent documents 5 to 8 suggest beamforming that is the proper linear combination of the ZF and MRT.
In non-patent document 5, it has been shown that a simple linear-type combination of egoistic beamforming and altruistic beamforming can achieve Pareto optimality in MISO (Multiple-Input Single-Output). Pareto optimality for a transmitter that is available for channel state information (CSI) partially in a MISO system has been disclosed in non-patent document 6. A MIMO system using a game-theoretic point of view has been disclosed in non-patent document 7. These non-patent documents achieve performance close to the optimal performance by using Bayesian games that allow each of the base station (BS) to operate semi-distributed. Non-patent document 8 proposed a beamforming scheme that uses a degree of the interference for a bargaining value, taking into account all instantaneous values and statistical values of the CSI. Non-patent document 9 proposed an inter-cell cooperative beamforming based on the virtual-SINR.